A Fischer-Tropsch (FT) process comprises the hydrogenation of CO in the presence of a catalyst based on metals, such as Fe, Co and Ru. The products formed from this reaction are water, gaseous, liquid and waxy hydrocarbons which may be saturated or unsaturated. Oxygenates of the hydrocarbons such as alcohols, acids, ketones and aldehydes are also formed.
A heterogeneous Fisher-Tropsch process may be conveniently categorised as either a high temperature Fischer-Tropsch (HTFT) process or a low temperature Fischer-Tropsch (LTFT) process. The HTFT process can be described as a two phase Fischer-Tropsch process. It is usually carried out at a temperature from 250° C. to 400° C. and the catalyst employed is usually an iron-based catalyst. Generally, the process is commercially carried out in a fluidised bed reactor.
The LTFT process can be described as a three phase Fischer-Tropsch process. It is usually carried out at a temperature from 220° C. to 310° C. and the catalyst employed is usually either a Co-based catalyst or a Fe-based catalyst. The conditions under which this process is carried out, results in the products being in a liquid and possibly also in a gas phase in the reactor. Therefore this process can be described as a three phase process, where the reactants are in the gas phase, at least some of the products are in the liquid phase and the catalyst is in a solid phase in the reaction zone. Generally this process is commercially carried out in a fixed bed reactor or a slurry bed reactor.
It is well-known that HTFT synthesis is preferred for the production of high value linear alkenes, and iron catalysts, operating at high temperatures in fluidised bed reactors, remain the catalysts of choice. LTFT synthesis using iron catalysts are usually the synthesis procedure of choice for the conversion of coal-derived synthesis gas (H2 and CO) to hydrocarbon products.
It is well known that in LTFT, especially in the production of heavy hydrocarbon products, a common problem is the relative slow synthesis rate and short catalyst lifetime. It is normal practise to try and solve such problems by an increase in reaction temperature to increase reaction rate, but this has lead to an increase in lighter hydrocarbon production (notably methane) as well as catalyst deactivation that results in a short catalyst lifetime.
Most procedures for preparing an iron based catalyst for FT synthesis produces a non-reduced catalyst wherein at least some (usually most) of the iron in the catalyst is in a positive oxidation state. In order to provide a catalyst which is active in FT synthesis (an activated FT catalyst) the catalyst has to be reduced to convert iron in the positive oxidation state to iron in a zero oxidation state.
Catalysis Today 36 (1997) 325; Canadian J. Chem. Eng., 74 (1996) 399-404; Applied Catalysis. A: General 186 (1999) 255-275; Journal of Catalysis 155, (1995) 366-375 and Energy and Fuels, 10 (1996) 921-926 describe different catalyst activation procedures and their influence on FT synthesis. The influence of different reducing gasses (H2, CO, or a combination of H2 and CO) used during activation is disclosed. Reduction at different pressures and temperatures are also disclosed. However, none of the documents disclose the activation conditions of the present invention and little emphasis is placed on the combination of the activation of the catalyst and the conditions under which the FT process in respect of activity and stability.
It has now been found that a Fischer Tropsch synthesis carried out under the conditions of the present invention together with the conditions according to the invention under which the Fischer Tropsch synthesis catalyst is activated results in an increase in Fischer Tropsch activity with little or no loss in respect of the stability of the catalyst.